Stem cell researchers in the UK are looking to use animal eggs as "hosts" to grow human cells.

A Chinese team has already claimed to have created human embryonic stem cells using rabbit eggs which had had their genetic material removed.

British experts said the option had to be considered because there were too few human eggs for research.

Scientists say cells created using animal eggs would only be used in the lab, but critics condemned the plan.

I think it is right to feel uncomfortable about creating some type of hybrid

Josephine Quintavalle, Comment on Reproductive Ethics

The recent controversy over South Korean researcher Hwang Woo-suk has added impetus to the need to find new sources of eggs, scientists say.

An investigation found he had faked cloning research; but it was also found that members of his team had donated eggs, against regulations.

Scientists in the field are concerned these events might deter women from donating their eggs for other research.

The aim behind Dr Hwang's research was to create cloned human embryos, and then develop stem cell lines from them - pointing the way towards individualised treatments in the future.

But for him - and other stem cell researchers - one of the key problems is accessing enough human eggs for their work, as it can be very difficult to fertilise them in order to get the early cell clusters from which stem cells - which can become any cell in the body - can be derived.

'No rabbit material'

In 2003, Dr Huizhen Sheng at Shanghai University carried out a study where human skin cells were placed into rabbit eggs.

A hundred were allowed to develop for several days before they were destroyed in order to retrieve the embryonic stem cells.

They said in a paper in the journal Cell Research that these were then able to become different types of cells, such as muscle cells.

Professor Chris Shaw, a neurologist from King's College London - along with Professor Ian Wilmut, whose team created Dolly the sheep - has been given permission to clone early stage human embryos to study motor neurone disease (MND).

The team is not planning to use animal eggs, but Professor Shaw said it was an option that researchers in the field had to consider.

"Hwang had 2,000 human eggs and he still failed to derive stem cell lines. We need to think of alternatives.

"Rabbit eggs are readily available, and we could do a lot of experiments and hone techniques.

"Stem cell lines from work using rabbit eggs might be just as informative."

Professor Shaw added: "There would be no rabbit genetic material used - you would be using the egg as a surrogate."

'No applications'

However, Josephine Quintavalle, of the campaign group Comment on Reproductive Ethics, said: "The question is 'what would be created?'

"I think it is right to feel uncomfortable about creating some type of hybrid.

The resulting embryo would be almost indistinguishable from a human embryo

Human Fertilisation and Embryology Authority guidance

"And I have suspicions of how useful it would really be for research."

Dr Chris O'Toole, Head of Research Regulation at the Human Fertilisation and Embryology Authority, said: "The issue of mixing human and animal material is complex and not thoroughly and explicitly dealt with under the current legislation.

"However, the HFEA has looked at this issue last September and concluded that any research that involves putting a human cell's nucleus into an animal egg would require a licence from us.

"As with all research involving human embryos, the research team would have to show that the research is both necessary and desirable, and that any embryo created could not be allowed to develop for longer than 14 days or be implanted in a woman."

Dr O'Toole said a specialist panel at the HFEA had looked at the issue of such hybrids and concluded: "the resulting embryo would be almost indistinguishable from a human embryo".

One other option is to ask women not going through IVF to donate their eggs for stem cell research.

Professor Alison Murdoch, a member of the team from the University of Newcastle which has successfully cloned a human embryo, is exploring the idea of such "altruistic" donation.

However, this would raise ethical and cost issues, which are currently being considered by the HFEA.